With the development of communications technologies, the gigabit copper wire (G.fast) leads a rate of copper wire access into a gigabit era. The G.fast is an access technology for ultra-high-rate bandwidth transmission over short-distance twisted pairs. An initial stage of a high frequency band of the G.fast uses 106 MHz and may be expanded to 212 MHz, and the G.fast with a higher frequency can obtain broader bandwidth. However, a higher signal frequency indicates a shorter transmission distance and higher costs and power consumption. In terms of uplink and downlink rate division, an FDD frequency division manner similar to that of the VDSL2 is not used in the G.fast; instead, a TDD time division multiplexing manner is used, and different time windows are used and assigned to uplink and downlink traffic.
A signal frequency used in the G.fast technology, compared with as existing DSL technology, is greatly expanded to a high frequency, but, at the same time, crosstalk between different ports of a bundle of cables is much severer than that of the existing DSL technology. Whether terminating impedance of a user port connected to a cable is matched not only affects mutual crosstalk between another wire pair, but also affects a transmission characteristic of two ends of the another wire pair in the cable. It can be known from the definition of impedance matching that impedance matching indicates specific cooperation relationships among load impedance, cable characteristic impedance, and impedance inside a signal source in a signal transmission process. Therefore, it is necessary to keep port impedance basically unchanged regardless of whether the port is in a transmitting, receiving, active, or inactive state.
In the prior art, a user port is connected through an interface circuit to a cable using the G.fast technology. In order to reduce power consumption for the interface circuit, when a signal transmitted by using the cable is received or the port is in an inactive state, a transmission-related device of the interface circuit stops working according to a characteristic that the TDD time division multiplexing is used in the G.fast technology. Similarly, when a signal is sent to the cable by using the cable, a reception-related device of the interface circuit stops working. However, because of a defect in design of a G.fast interface circuit in the prior art, when a device in the interface circuit in the technology is in a different state such as a working state or a working stopped state, an impedance characteristic of the device changes, resulting in that terminating impedance of the user port cannot be matched, thereby causing a change in mutual crosstalk between a wire pair and a change in a transmission characteristic of two ends of the wire pair. As a result, transmission stability of the cable decreases.